The present invention relates to a mobile station apparatus and a base station apparatus for performing data transmission of a CDMA (Code Division Multiple Access) system and a communication method using these apparatus.
In a mobile communication system of third generation, a CDMA system is planning to be used as a radio access system. Generally, in the CDMA system, a spreading code peculiar to each radio cell (radio base station) is used to a radio signal (forward link), which each radio base station transmits. Also, a spreading code peculiar to each mobile station is used with respect to a radio signal (reverse link), which each mobile station transmits.
In the CDMA system, an advantage in which the same frequency can be used between the adjacent radio cells (hereinafter referred to as cell as required) is exploited and a soft handover system is adopted as a handover system, which is used when the mobile station moves between cells. The soft handover system combines radio signals from the base station of the mobile source cell and the base stat on of the mobile destination cell.
In order to execute such a handover, the mobile station need to perform a peripheral cell search for searching the base station of the mobile destination cell. Each base station always transmits a radio channel, which is called perch channel. The perch channel is transmitted from each cell (base station) using an inherent spreading code.
Therefore, each station notifies the mobile station of the spreading code, which is used in the peripheral cells, of the self-station as broadcast information. On the other hand, the mobile station periodically detects a link quality (reception SIR (signal interference ratio) of the radio channel (perch channel of the peripheral cell) using the notified spreading code. Or, the mobile station periodically detects reception Eb/IO (a ratio of energy of a received signal per one bit to interference, a heat control power spectrum density ratio) Then, the mobile station notifies the network side of the detection result. By performing the so-called peripheral cell search, the network can determine to which cell the mobile station is moving.
FIG. 1 is a view showing an outline structure of a conventional mobile communication system, particularly the structure relating to the handover. The mobile communication system mainly comprises a mobile station, radio base stations and a control station.
A mobile station M mainly comprises a radio receiving and transmitting section 11, a message decoder 12, a spreading code storing section 13, a reception quality detecting section 14, a peripheral cell control section 15, and a message generator 16. The radio receiving and transmitting section 11 receives and transmits a radio signal between the mobile station M and the radio base station. The message decoder 12 decodes control information contained in the signal received by the radio receiving and transmitting section 11. The spreading code storing section 13 stores spreading codes of an object to be subjected to the peripheral cell search designated from the network side, that is, the spreading codes used in the peripheral cells. The reception quality detecting section 14 detects the link quality of the signal received by the radio receiving and transmitting section 11. The peripheral cell search control section 15 sets the spreading codes stored in the spreading code storing section 13 to be used in the peripheral cells with respect to the reception quality detecting section 14, sequentially. Then, the peripheral cell search control section 15 manages the reception quality detecting cycle, and the number of times of detection, and provides an averaging processing to the detection result. The message generator 16 generates a message, including control information, to the network.
The radio base stations B mainly comprise a plurality of radio receiving and transmitting sections 17, a wire receiving and transmitting section 18, and a radio control section 19. The radio receiving and transmitting sections 17 receive and transmit the radio signals between the mobile station and the radio base station. The wire receiving and transmitting section 18 receives and transmits control signals, that is, a mobile station control signal and a radio base station control signal between the control station and the radio base stations B via radio. The radio control section 19 controls the radio receiving and transmitting sections 17 in accordance with a designation from the control station. A plurality of radio base stations Ba, Bb, and Bc thus structured is provided.
The control section C mainly comprises a station site storing section 20, a peripheral cell information generator 21, a wire receiving and transmitting section 22, a message decoder 23, a mobile direction determination section 24, a handover control section 25, and a message generator 26. The station site storing section 20 stores the inherent spreading code, which is used by the radio base station under the control of the self-station. The peripheral cell information generator 21 generates peripheral cell information for each radio cell based on station site information stored in the station site storing section 20. The wire receiving and transmitting section 22 receives and transmits control signals, which are received and transmitted from/to the radio base stations Ba, Bb, and Bc. The message decoder 23 decodes control information from the mobile station. The mobile direction determination section 24 determines a mobile destination cell of the mobile station based on a peripheral cell search result report from the mobile station and information from the station site storing section 20. The handover control section 25 designates the execution of the handover to the mobile station M and the radio base stations B when it is determined that the handover is needed based on information from the mobile direction determination section 24. The message generator 26 generates a message including control information on the mobile station M and the radio base stations B.
Next, an operation of the handover of the above-structured conventional mobile communication system will be explained with reference to FIG. 2.
The spreading code which each radio base station uses as a reverse link is set in advance, and information on the spread code setting is managed by the station site storing section of the control station.
The radio control section 19 of the radio base station Ba uses a predetermined spreading code Ca as a forward link, and the mobile station M uses a spreading code Cm peculiar to the mobile station as a reverse link.
The mobile station M is in a cell area. The mobile station M receives a peripheral cell spreading code notice from the radio base station Ba offering a service of the cell a, so that the spreading codes used in the peripheral cells are stored in the spreading code storing section 13 (ST1).
The peripheral cell search control section 15 sets a cell search execution cycle xcex94t in a timer T (ST2). Then, when the timer counts up (ST3), the spreading codes, which are used by the peripheral cells stored in the spreading code storing section 13, are sequentially set in the reception quality detecting section 14. The reception quality detecting section 14 detects the reception quality from the peripheral cells of the radio base station Ba (ST4).
Next, the peripheral cell search result is reported to the network. The above explained a case in which the peripheral call search result was reported to the network every time when each peripheral cell search was executed. However, the result is not necessarily reported to the network every time.
The mobile direction determination section 24 of the control station C first determines the mobile direction based on the peripheral cell reception quality report from the mobile station M (ST5) to determine whether or not the cell movement is needed. If it is determined that the cell movement is not needed (ST6) control information is not transmitted to the mobile station. For this reason, the reception quality from the periphery cell is detected again (ST4).
If it is determined that the cell movement is needed as a result of determination of the mobile direction (ST5) that is, the mobile station M is moving to a cell b from the cell a, the following operation is executed. More specifically, the start of the forward link transmission to the mobile station M and that of the reverse link reception from the mobile station M are designated to the radio base station Bb offering a service of the cell b. This operation is called soft handover designation.
After that, the mobile station M is moved to the cell through the soft handover state (ST8). Thereafter, the control station notifies the mobile station of the spreading code used in the peripheral cells of the cell b.
Thus, in the case in which the same frequency is used between the adjacent cells, the handover is executed by performing the peripheral cell search in the mobile station and by reporting the result to the network.
The CDMA system can cover the whole service area using only a single frequency since the same frequency can be used between the adjacent cells. However, in the CDMA system, there is an upper limitation since the number of users, which can be contained by use of one frequency, is determined by an amount of interference.
On the other hand, in order to increase the number of users per cell, a plurality of frequencies must be used. In the service area to which the mobile communication system is applied, there are various areas including an extremely high traffic area such an urban area to a low traffic area such as a suburban area. The frequency suitable for an amount of traffic, which is required in each area, is preferably used. Therefore, in the service area, the handover must be realized after the frequency to be used in each area can be freely set.
However, the mobile communication system is predicated on the point that the same frequency is used in the entire service area. If the mobile communication system is used in the service area having a different frequency, there occurs a problem in which the handover cannot be satisfactorily performed between the cells each having a different frequency.
The present invention has been made in consideration of the above-mentioned problem. An object of the present invention is to provide a mobile communication system capable of satisfactorily performing an handover even in service areas each having a different frequency, and to provide a mobile communication method.
The above object can be achieved by changing the using frequency in advance when a mobile direction of a mobile station M is determined in a network side (control station) and it is further determined whether or not the mobile station M moves out of the same frequency area, and when the mobile station M moves out of the same frequency area.